Electron Spin Resonance of Fe³⁺ in SrTiO₃ with Special Reference to the 110°K Phase Transition

The electron spin resonance of F 3+ in single crystals of SrTiO 3 has been studied in the low temperature phase below 110°K. The resonance lines exhibited small splitting in addition to the usual tetragonal splitting. The orientation dependence of the splitting was measured at 77°K and at 4.2°K. The whole spectrum is explained to be due to the rotation of oxygen octahedron in the (100) plane, the angle of rotation being 1.4° at 77°K and 2.1° at 4.2°K. Corresponding splitting was also observed for the case of the `strong axial field' spectrum due to Fe 3+ accompanied by the nearest neighbor oxygen vacancy. The observed angles of rotation are about 30 percent smaller than those of the former case. The crystal structure deduced from the atomic positions of oxygen ions will be D 4 h 16 in the low temperature phase. Correlation with the resonance data of Gd 3+ in SrTiO 3 is discussed.